1. Field of the Invention
This invention relates to the technical field of high-density and large-capacity memory devices which use, for example, the principle of a scanning tunneling microscope (hereinafter termed an STM).
2. Description of the Prior Art
Heretofore, semiconductor memories comprising semiconductors and magnetic memories comprising magnetic materials have been mainly used as memories. Recently, however, in accordance with the development of laser techniques, inexpensive high-density recording media comprising optical memories which use organic thin films made of organic dyes, photopolymers or the like have appeared.
In addition, STM's which can directly observe the electronic structure of surface atoms of a conductor have been developed (G. Binnig et al. Phys. Rev. Lett. 49, 57 (1982)). The STM has the advantages that a real-space image of a material, irrespective of crystalline or amorphous, can be measured with high resolution, a sample can be observed at low power without damaging it by current, and since the apparatus operates even in air, it can therefore be used for various materials. Accordingly, a wide range of applications is expected. The STM utilizes the phenomenon that a tunnel current flows when a metal probe (probe electrode) and a conductive substance are brought as close as about 1 nm (nanometer) while applying a voltage between them. The tunnel current is very sensitive to a change in the distance between the probe and the substance. By scanning the probe so as to maintain the tunnel current at a constant value, it is even possible to read various kinds of information with respect to the entire electronic cloud of a real space. At that time, resolution in the direction of the plane of the substance is about 0.1 nm.
Accordingly, by applying the principle of the STM, it is possible to sufficiently record and reproduce high-density data in the atomic order (in the order of sub-nanometers). For example, in a recording/reproducing apparatus disclosed in Japanese Patent Application Public Disclosure (Kokai) No. 61-80536 (1986), a writing operation is performed while removing atomic particles adsorbed on the surface of a medium using an electronic beam or the like, and the written data is reproduced using an STM.
Methods exist in which an STM performs recording and reproducing operations using a thin-film recording layer of a material having a memory effect with respect to voltage-current switching characteristics. Examples of recording layer materials include a .pi.-electron-type organic compound, a chalcogenide compound or the like, and are disclosed, for example, in Japanese Patent Application Public Disclosures (Kokai) Nos. 63-161552 (1988) and 63-161553 (1988). According to such methods, large-capacity recording and reproducing operations on the order of 10.sup.12 bits/cm.sup.2 become possible assuming the size of recording bits of 10 nms.
In using such recording/reproducing methods, in order to reproduce data with an excellent S/N ratio, it is in general necessary to perform so-called tracking control wherein a probe electrode is moved along a recorded signal string while performing feedback control, since recording bits are very small. For example, a method has been proposed in Japanese Patent Application Public Disclosure (Kokai) No. 1-107341 (1989) wherein a V-shaped groove is previously formed on the surface of a recording medium, and a probe electrode is controlled so as to be always situated in the center of the groove. On the other hand, methods which do not require a tracking operation have been proposed. For example, a method is disclosed in Japanese Patent Application Public Disclosure (Kokai) No. 2-50333 (1990), wherein, in a reproducing operation, information is read by scanning a recorded signal string with a probe electrode with a density higher than the recording density without performing a tracking operation, and recorded information is reproduced using a pattern recognition technique.
However, control is complicated in the above-described methods which use a tracking operation, and a complicated pattern recognition process requiring a long processing time is necessary in the above-described method which uses pattern recognition without using a tracking operation.